thanks chuck very helpful

Hopefully someone can help me.  I just found "The Fast Mail" G Scale train set that my mom used to use that hasn't been used in at least 10 years.  Those are the only train cars that I have and would like to add more.  I tried to look on the site for additional cars but am a little confused.  Can I just get a car that is characterized G Scale ?  Some look to me to be different sizes but say G scale.  Any information would be helpful.  Thanks